Create infix_to_prefix.c

By Shourya Gupta
A simple program using user defined function to  Evaluate an expression given by the user in infix to prefix using C languague

C/infix_to_prefix.c

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+ /* infix to prefix conversion*/
+
+#include<stdio.h>
+#include<string.h>
+#define MAX 20
+char stack[MAX];
+int top=-1;
+void push(char item)
+{
+ top++;
+ stack[top]=item;
+}
+char pop()
+{
+ char a;
+ a=stack[top];
+ top--;
+ return a;
+}
+int prcd(char symbol)
+{
+ switch(symbol)
+ {
+ case '+':
+ case '-': return 2;
+ case '*':
+ case '/': return 4;
+ case '^':
+ case '$': return 6;
+ case '(':
+ case ')':
+ case '#': return 1;
+ }
+ return 0;
+}
+int isoperator(char symbol)
+{
+ switch(symbol)
+ {
+ case '+':
+ case '-':
+ case '*':
+ case '/':
+ case '^':
+ case '$':
+ case '(':
+ case ')': return 1;
+ default: return 0;
+ }
+}
+void infix_to_prefix(char infix[], char prefix[])
+{
+ int i,symbol,j=0;
+ char test[MAX];
+ infix=strrev(infix);
+ stack[++top]='#';
+ for(i=0;i<strlen(infix);i++)
+ {
+ symbol=infix[i];
+ if(isoperator(symbol)==0)
+ {
+ prefix[j]=symbol;
+ j++;
+ }
+ else
+ {
+ if(symbol==')')
+ {
+ push(symbol);
+ }
+ else if(symbol=='(')
+ {
+ while(stack[top]!=')')
+ {
+ prefix[j]=pop();
+ j++;
+ }
+ pop();
+ }
+ else
+ {
+ if(prcd(symbol>=prcd(stack[top])))
+ {
+ push(symbol);
+ }
+ else
+ {
+ while(prcd(symbol)<prcd(stack[top]))
+ {
+ prefix[j]=pop();
+ j++;
+ }
+ push(symbol);
+ }
+ }
+ }
+ }
+ while(stack[top]!='#')
+ {
+ prefix[j]=pop();
+ j++;
+ }
+ prefix[j]='\0';
+ prefix=strrev(prefix);
+}
+
+/*The driver code*/
+int main(void)
+{
+ char infix[20],prefix[20];
+ printf("\nenter infix expression:");
+ scanf("%s",infix);
+ infix_to_prefix(infix,prefix);
+ printf("\nequivalent prefix expression:");
+ puts(prefix);
+ return 0;
+}